Embodiments of the invention relate generally to medical imaging devices and other systems employing superconducting magnet systems and, more particularly, to a system and method for ramping-down a superconducting magnet in an automated fashion for servicing.
As is known, a coil wound of superconductive material (a magnet coil) can be made superconducting by placing it in an extremely cold environment, (e.g., −269° C. or 4 K). For example, a coil may be made superconducting by enclosing it in a cryostat or pressure vessel containing a cryogen. The extreme cold enables the coil wires to be operated in a superconducting state. In this state, the resistance of the wires is practically zero. To introduce a current flow through the coils, a power source is initially connected to the coils for a short time period. In the superconducting state, the current will continue to flow through the coils, thereby maintaining a strong magnetic field. In other words, because superconductive windings offer little to no resistance to electrical current flow at low temperatures, the superconducting state of the magnet is persistent. The electric current that flows through the superconducting magnet is maintained within the magnet and does not decay noticeably with time.
Superconducting magnets have wide applications in the field of magnetic resonance imaging (“MRI”). In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrically shaped cryogen pressure vessel containing a cryogen, such as liquid helium. The cryogen vessel is contained within an evacuated vessel and formed with an imaging bore in the center. The main magnet coils develop a strong magnetic field in the imaging volume of the axial bore that, when combined with controlled gradient magnetic fields and RF pulses, act to generate a signal from a patient may be received and processed to form an image. In existing MRI systems, a main magnetic field of 3 Tesla may be desired to produce vivid images.
Occasionally, it may be desirable or necessary to de-energize the superconducting magnet. For example, when components of the cryogenic cooling system degrade, when leaks within the cryogen (helium) vessel are found, or when other maintenance on the MRI system is desired, it may be necessary to de-energize the magnet system. Deactivation may require a “ramp-down” of the magnet, where current must be slowly lowered in the magnet to weaken the magnet field strength. These ramp-downs are often manually intensive, time consuming, and expensive. Typically, it can take 34 hours to de-energize a 3.0 T magnet using conventional techniques, during which time a magnet engineer must be present to oversee the ramp-down procedure. Furthermore, as external leads are connected to the magnet during the ramp-down, heat is added to the magnet thereby causing an increased rate of boil-off of the liquid helium as it boils into helium gas. The helium gas may be vented to the atmosphere, which can add significant costs to the ramp-down operation since the vented helium gas may not be recoverable from the atmosphere for conversion back into a liquid. Accordingly, a portion of the cryogen may need to be replaced.
Additionally, the ramp-down of the magnet may, if performed improperly, cause damage to the magnet system. For example, if current in the magnet is ramped down too quickly, a phenomenon of “quenching” may occur, in which a localized portion of the magnet increases in temperature and loses superconductivity. This localized increase in temperature can burn or damage the superconducting coils of the magnet. In addition, the rapid decrease in the molecular density within the cryogen vessel resulting from a sharp temperature rise sharply reduces the insulating ability normally provided by the helium gas resulting in possible voltage breakdown through the gas which can seriously damage the various coils and associated control circuitry of the elements within the cryogen vessel.
Thus, for the aforementioned reasons, a field engineer may typically be generous in the amount of time used to ramp-down the magnet current. Unfortunately, however, a conservative ramp-down results in longer ramp times, increased downtime, increased maintenance costs, and so forth. Thus, conservative magnet ramp-down techniques may increase the cost of operating and maintaining an MRI system and may decrease the availability of the MRI system.
It would therefore be desirable to have a system and method capable of reducing MRI periods of deactivation, costs, and so forth. Particularly, there is a need to reduce the ramp-down time of the superconducting magnet while maintaining equipment integrity and while reducing helium loss, equipment downtime, and maintenance costs.